Although carotenoid pigments have been demonstrated to be effective quenchers of singlet oxygen in vitro, and their deficiency in vivo is frequently associated with the appearance of an aerobic photosensitivity, direct evidence that carotenoid pigments function in vivo by quenching singlet oxygen is still lacking.We propose to look at carotenoid isomerization, both in vitro and in vivo as a means of identifying singlet oxygen quenching. Both photochemical and chemical sources of singlet oxygen will be employed.